There exists already a rather large number of techniques for hydrolyzing cellulose to a minor or larger extent by means of hydrochloric acid in concentrated or diluted solutions. These methods, among which there can be mentioned the BERGIUS, HERENG, PRODOR and other processes are disclosed for instance in the following references. Wood Chemistry, Process Engineering Aspects (1965), NOYES DEVELOPMENT CORP., Park-Ridge, N.J. 07656 USA; U.S. Pat. Nos. 2,951,775; 2,959,500; 9,974,067; 2,752,270; 2,778,751; 2,945,777; 3,212,932; 3,212,933; 3,251,716; 3,266,933 and 3,413,189.
However, when one uses solutions of hydrochloric acid, he must use rather high weight ratios of HCl to cellulose which requires, to ensure that the operation is profitable, that means for recovering and recycling this acid be provided. Now, such means are de facto not economical because of the additional equipment involved and, besides, there is a consecutive increase of the corrosion problems in connection with the use of such acids. Hence, means have been sought to remedy these drawbacks by decreasing the total amount of HCl put into work and, as a consequence, to increase its concentration at sites very close to the fiber to be hydrolyzed. Thus, a method is disclosed in U.S. Pat. No. 1,806,531 (GOGARTEN et al) in which cellulose containing materials are saturated with dry HCl (in compressed or liquid form) below zero degree C under pressure in an autoclave whereby decomposition of the material occurs at a temperature of 0.degree. C. or below. Then, steam is introduced into the mass for raising the temperature to about 60.degree.-75.degree. C. and for effecting the saccharification of the decomposed cellulosic product. This method is economically interesting in some aspects since it uses a relatively low HCl cellulosic product weight ratio and also enables to recover part of this acid in highly concentrated form after saccharification. However, this advantage is offset by the necessity to use pressure equipment which is extremely difficult and costly to operate in the presence of compressed gaseous or liquid anhydrous HCl because of corrosion problems. Another disadvantage is that the addition of steam (i.e. a raise in temperature to relatively high values) is known to cause the decomposition of the sensitive sugars formed from wood, namely the pentoses which then turn into a black resin. Another disadvantage is the requirement that the solid comminuted cellulosic material in the autoclave be cooled to zero degree C. or below which is not easy to achieve in all cases and, more particularly, when using rotating pressure equipment. Finally, operating the decomposition of the cellulosic material at low temperatures such as 0.degree. C. or below is not efficient since then the reaction rate is very slow and mixing is very poor as the cold mixture of HCl and cellulosic material is very stiff.
In another reference, i.e. U.S. Pat. No. 1,677,406 (PERL), there is disclosed a method for the saccharification of cellulose bearing products in which some of the above drawbacks are avoided. In this method, the cellulose product (wood chips) is progressively driven in a helical conveyor while a refrigerated mixture of dry HCl and a carrier gas is fed at counter-current relative to the displacement of the cellulose from the downstream side of the conveyor. The gases continuously travel through the conveyed material whereas the HCl is progressively adsorbed therein and the exhausted gases are then removed from the conveyor, replenished with fresh HCl and recycled in the system. This arrangement enables to have the fresh gases (with high HCl concentration) to first meet the material with the highest HCl saturation which minimizes undesirable local temperature jumps due to the heat produced by the dry HCl interacting with the fiber. This method is attractive but suffers from several drawbacks which prohibit profitable industrial application. Such drawbacks are, for instance, the extreme complexity of the mechanism of drive and valves for accurately controlling the moving of the comminuted solid and the concentration of gases at all stages of the process, both factors which are intimately interdependant, the required presence of very efficient carrier gas coolers which are not economical to run, the requirement to maintain moving parts under gas tight conditions since the equipment must operate under positive gas pressure (HCl gas is very corrosive to joints) and the rate of the overall reaction that will be relatively slow because of gas dilution as compared with methods using undiluted HCl gas.
In another method, the CHISSO process (Chemical Economy and Engineering Review II (6) (1979), 32), cellulose or wood particles, preferably prehydrolyzed with diluted acid, are impregnated with concentrated aqueous HCl solution until the water content of the mass is from 50% to 70% by weight, then, with the knowledge that the saturation concentration of an aqueous solution of HCl is inversely proportional to temperature, said mass soaked with aqueous acid is treated, below 10.degree. C., with a current of HCl gas for increasing the HCl concentration in the solution until the cellulose of the impregnated mass will dissolve (indeed, the cellulose only dissolves significantly in HCl solutions when the concentration of HCl therein is or exceeds 39% by weight). Then, the whole material is heated to 35.degree.-50.degree. C. for effecting the hydrolysis of this cellulose in a relatively short time of 10 to 30 minutes. In the course of this heating, it is necessary to add some more water to compensate for the evaporation losses in the mass during the hydrolysis in which hydrosoluble oligomer polysaccharides are formed. Then, the excess of acid is separated by means of a current of hot air or HCl and recovered, the operation being performed as quickly as possible to minimize some possible decomposition of the monomeric sugars already made free during the said hydrolysis. Finally, there is added a relatively large volume of water to the mass for dissolving it completely and for carrying out the post-hydrolysis of the oligosaccharides into monomeric sugars, such post-hydrolysis being effective only in a solution of relatively low acid concentration (about 1-5%).
Such a method indeed enables to significantly reduce the quantity of acid put into operation relative to the older methods. It however presents some disadvantages which should be desirably remedied and which are as follows:
(a) the cellulosic materials should preferably be prehydrolyzed before saccharification by gaseous HCl; indeed, it is preferable to eliminate beforehand the pentoses which are easily separated by hydrolysis with diluted acid to prevent them from being possibly decomposed at the highest temperatures of the above-mentioned range (50.degree. C.), PA0 (c) the obligation to compensate by a further addition of water the losses due to evaporation during the hydrolysis operation at 35.degree.-50.degree. C. is an undesirable complication, PA0 (d) the recovery of the gaseous acid is unseparable from some decomposition of the reducing sugars; this decomposition is slight but still significant at the temperature prevailing during this recovery. PA0 (a) one impregnates at a relatively low temperature (i.e. a temperature sufficiently low for ensuring that the rate of hydrolysis is still unsignificant thus preventing unexpected local overheating and possible decomposition of the product) a humid comminuted mass of cellulose or cellulose containing matter with gaseous HCl so as to cause the water contained in this mass to get progressively saturatively loaded with hydrochloric acid; PA0 (b) one heats the mass thus impregnated for triggering a first hydrolysis reaction leading to the conversion of the said mass into oligosaccharides and other reducing sugars; PA0 (c) one removes part of the acid in the form of gas for the purpose of its recovery; and PA0 (d) one adds to the mass thus prehydrolyzed an amount of water sufficient to complete the hydrolysis and he heats to convert the available oligosaccharides into monomeric sugars. The method of the invention however distinguishes from the prior-art by the following fundamental difference: the temperature at which the first hydrolysis is started is very close to 30.degree. C., i.e. only slightly above or below said value (e.g. comprised between 28.degree. and 33.degree. C.). Thus, when heated (or allowed to come) to this temperature, the excess of HCl gas having been added under cooling to the water of the mass, this being for instance up to saturation, escapes therefrom in the form of micro-bubbles thus providing a "brewing" action that considerably improves the efficiency of the hydrolysis operation during which cellulose is converted into oligosaccharides and which can be thus carried out with an excellent yield and at a relatively moderate temperature even in the case of a material not prehydrolyzed and not delignified beforehand. In practice, one can either maintain the temperature between about 30.degree. and 33.degree. C. or, after the reaction has started together with the above mentioned gas evolution, he can heat to a higher temperature (particularly in the absence of the decomposable pentoses, i.e. when using a starting cellulose from which the hemicellulose has been removed beforehand) so as to further accelerate the first hydrolysis step. In these conditions, this hydrolysis can be accomplished within a period comprised between a few minutes and about 2 hrs. If pentoses are present, the hydrolysis temperature will preferably not exceed about 40.degree. C.; in the absence of pentoses, the temperature can go higher, e.g. to 70.degree. or even 80.degree. C. although at the higher end of this range hexoses are also subject to some degree (not too much, fortunately) of decomposition (dark resins). It is noted that, during this hydrolysis, the oligosaccharides formed dissolve, all or in part depending on the water available in the mass, in the acid solution with which the latter is impregnated thus forming highly concentrated solutions, for instance of the order of 500 g/l, the total of the hydrosoluble dissolved and not dissolved substances being actually susceptible to be still much higher, e.g. 1000 to 1500 g/l.
(b) the mass should be impregnated beforehand with concentrated acid solution prior to the treatment with gaseous HCl. Thus, it is not possible to completely avoid the initial use of concentrated aqueous HCl solutions,